Muscle glycogen debranching enzyme (amylo-1,6-glucosidase/4-alpha-glucanotransferase) is the only multicatalytic site mammalian (eucaryotic) enzyme to be reported that is active as a monomer. The enzyme, a single polypeptide molecule with a molecular weight of 160-170,000, consists of two separate enzymatic activities which have different modes of action. The debranching enzyme exists along with phosphorylase and its activating enzyme system as a component of a protein-glycogen multi-enzyme complex in muscle which is involved with glycogen metabolism and linked to the regulation of muscle contraction. Investigation of the relationship between the two debrancher activities has led to a mechanism of how the enzyme functions in debranching and given some insight into how it is constructed and the relationship it has with phosphorylase as a component of the protein-glycogen complex. The present study involves an investigation of the catalytic mechanism of the debrancher as a protein molecule to determine how the two activities (glucosidase and transferase) relate to each other as structural components of the polypeptide. Based on the evidence concerning the structural requirements of reversible inhibitors that act as glucose "transition state" analogs and an active-site-directed inhibitor, a characterization of the glucosidase active site will be undertaken to determine the catalytic groups involved and the nature of an active glycosyl-enzyme intermediate. This will clarify the glycosyl receptor requirements of the glucosidase site and the possible role of polyamine metabolites, which inhibit the glucosidase and have been implicated in cystic fibrosis and which may act as intracellular hormonal messengers and/or function as antagonists in the regulation of glycogen metabolism in muscle. This study will delineate the relationship between the structure and function of the debrancher as well as its role as a component of the glycogenolytic complex, and will elucidate another aspect of the control exerted on muscle glycogenolysis.